Add test to zmqpp::message move constructor/assignment operator.

These tests fail.

It shows that read_cursor is not properly
updated when moving messages. It can cause trouble with
using operator>>. See #73.

Also, when using operator= the moved-from message ends up with
parts from the moved-to message. This is incorrect.
Problem: zmqpp::message move-constructor and operator= misbehave.

See previous commit for tests explaining those problems.
Merge pull request #74 from xaqq/message_move

Message move